The subject invention relates to fluid or fixed bed catalytic oxychlorination of ethylene to produce 1,2-dichloroethane, commonly called ethylene dichloride (EDC) and relates specifically to improved copper catalysts and their use in ethylene oxychlorination reactions.
Catalysts for the production of chlorinated hydrocarbons by oxychlorination have been well established for a number of years. Conversion of ethylene (C.sub.2 H.sub.4) to 1,2-dichloroethane by oxychlorination is practiced in commercial installations throughout the world. The preferred method is a vapor phase reaction, over a fluidized catalyst bed, of a mixture of ethylene, hydrogen chloride (HCl) and oxygen or an oxygen containing gas (e.g., air). An example of the conditions required are described in U.S. Pat. No. 3,488,398 to Harpring et al.
A typical catalyst used in oxychlorination reactions comprises about 4% to 17% by weight of a copper compound. Typically, the copper compound is cupric chloride, as the active catalytic ingredient, deposited on particles of a fixed fluidizable support, such as silica, kieselguhr, clay, fuller's earth, or alumina. For use in non-fixed bed catalysis, the support should be readily fluidizable without excessive catalyst loss from the reaction zone, and have proper particle density, resistance to attrition and particle size distribution to be useful in the process. In oxychlorination processes most closely aligned to the present invention, an alumina support is employed which may be gamma alumina, alpha alumina, the so-called microgel aluminas or other forms of "activated" alumina. The standard fixed and fluid bed alumina-based oxychlorination catalysts can be improved upon in significant respects.
It is desirable for the oxychlorination catalyst to effect the highest possible yield of EDC based on ethylene (i.e., for the ethylene to be more completely converted to EDC, with less ethylene being reacted to carbon oxides or higher chlorinated materials). In the high volume business of manufacturing EDC, small increases in the efficiency of ethylene conversion to EDC are very valuable. For example, in a one billion pound per year EDC oxychlorination plant, an ethylene efficiency increase of only 1% can result in a savings of from about 0.5 to about 1.0 million dollars annually. Further, increased ethylene efficiency reduces the amount of by-products produced and the associated potential of release of hydrocarbons and chlorinated hydrocarbons to the environment.
Further, it is becoming much more desirable, for economic and environmental reasons, for the oxychlorination catalyst to also effect a high conversion of the hydrogen chloride (HCl) used in the reaction. Problems can arise when a higher than theoretical molar ratio of HCl to ethylene is used in an attempt to achieve higher ethylene conversions to EDC. Unconverted HCl must be neutralized using, for example, a caustic solution, and the resulting salt must be disposed. Also, higher levels of HCl in the process can lead to higher HCl "break through" downstream of the reactor which can cause corrosion problems. Hence, a modern oxychlorination process will attempt to operate at an HCl to ethylene molar ratio as close to, but not exceeding, the theoretical level of two-to-one (2:1) as possible in conjunction with high HCl conversion. In commercial practice in which ethylene is passed through/over the catalysts one time, the ratio is generally from about 1.93 to about 1.97. In the process where the unreacted ethylene is separated and then recycled, a lower ratio of from about 1.88 to about 1.92 can be employed. In either application, a combination of high HCl conversion and high ethylene efficiency is most desirable.
Lastly, typical cupric chloride on alumina fluid bed catalysts may exhibit a tendency to develop "stickiness" during the oxychlorination reaction at HCl to ethylene molar feed ratios greater than about 1.9. Catalyst stickiness, which is basically agglomeration of catalyst particles, may be a critical barrier to achieving optimum ethylene and HCl feedstock efficiencies in a fluid bed oxychlorination process. The highest ethylene efficiency from an oxychlorination catalyst requires operation with an HCl/ethylene molar feed ratio approaching, but not exceeding, the stoichiometric value of 2.0. However, as the HCl/ethylene feed ratio is increased above about 1.9 in a commercial process, standard fluid bed oxychlorination catalysts may become progressively more sticky. With increased catalyst stickiness, heat transfer characteristics of the fluid bed worsen, hot spots develop within the catalyst bed, feedstock conversions and yields decline, and, in extreme cases, the bed actually collapses and slumps, causing vapor channel passages through the bed. In commercial operation, upsets to the feedstocks, temperature variations, etc., can lead to an HCl/ethylene ratio above the preferred ratio; therefore, a high performance oxychlorination catalyst requires the ability to operate over a wide range of HCl/ethylene feed ratios (1.85-2.2). Other requirements for high performance catalysts are excellent fluidization and high conversions, yields, and efficiencies. This problem of catalyst stickiness and a device and means for its partial control are described in U.S. Pat. No. 4,226,798 issued to Cowfer et al. A method of controlling stickiness in standard oxychlorination catalysts is also described in U.S. Pat. No. 4,339,620, also issued to Cowfer et al. Although these devices and methods are helpful, it is more practical and efficient to employ an oxychlorination catalyst which does not develop stickiness during the reaction.
There are references which disclose the use of alkali metals, alkaline earth metals, or rare earth metals along with copper chloride. Although these catalysts are closer in composition to those of the present invention, improvements in composition and performance can still be obtained. None of these references teach or suggest the types and amounts of metals used to improve catalyst performance. Much effort has been put into the improvement of catalysts for oxychlorination of ethylene to form EDC. Due to the large volume of product produced, a small increase in efficiency can produce a large return in cost savings. Increasing the HCl conversion and ethylene efficiency will prove beneficial to the environment as well.
It is worthwhile to note the references most closely aligned with the catalyst and process of the present invention are U.S. Pat. No. 4,740,642 to Eden et al and U.S. Pat. No. 3,205,280. U.S. Pat. No. 4,740,642 relates to a catalyst composition comprising copper, an alkali metal salt and a rare earth metal salt. U.S. Pat. No. 3,205,280 discloses a catalyst composition of an Al.sub.2 O.sub.3 support (calcined at 900.degree. C. which substantially lowers its surface area) having thereon an alkali metal such as potassium chloride, and/or an alkaline earth metal, a transition metal such as copper, and/or a rare earth metal such as didymium. Both references require specific and limited ratios of alkali or alkaline earth metal to transition or rare earth metals.
The catalysts of the present invention are evaluated based upon a number of criteria: ethylene efficiency, ethylene conversion, HCl conversion, EDC selectivity, carbon dioxide and carbon monoxide selectivity, triane (1,1,2-trichloroethane) selectivity and fluidization quality for fluid bed catalysts. Ethylene and HCl conversion is simply a determination of the amount in mole % of reactant consumed in the reactor. The selectivity is the mole percent yield of pure product formed. The ethylene efficiency is the product of the ethylene conversion and the EDC selectivity, e.g., a 99% ethylene conversion and a 95% EDC selectivity would result in a 94% ethylene efficiency. Small increases in ethylene efficiency, as low as 0.5%, can result in a very large savings due to the large volume of product produced. Also, the reduction of wastes, e.g., over chlorinated by-products, such as triane (1,1,2-trichloroethane), can represent a big savings. These materials currently can cost a producer as much as $500 per ton to dispose in an environmentally safe manner. Therefore, reducing this by-product can save money as well as reduce the potential for pollution.